Move CONFIG_OF_LIBFDT to Kconfig

Move this option to Kconfig and tidy up existing boards.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/Kconfig b/lib/Kconfig
index c7eab46..d22a8e4 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -128,6 +128,16 @@
 	  - if errno is null or positive number - a pointer to "Success" message
 	  - if errno is negative - a pointer to errno related message
 
+config OF_LIBFDT
+	bool "Enable the FDT library"
+	default y if OF_CONTROL
+	help
+	  This enables the FDT library (libfdt). It provides functions for
+	  accessing binary device tree images in memory, such as adding and
+	  removing notes and properties, scanning through the tree and finding
+	  particular compatible nodes. The library operates on a flattened
+	  version of the device tree.
+
 source lib/efi/Kconfig
 
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